McDonald's is quite different than in the USA, since cows are  fed grass, not grain.  Plus some of them have the adorable McCafe coffee bar.

The McMagnifico or Quarter Pounder meal is A$11. 

Other cheap eats options include pretty good pizza, sandwiches or empanadas. There are lots of street vendors and little cafes where you can find a low-cost breakfast or lunch. E.g. cafe "Dulcinea" on Santa Fe Ave. offers a lunch of toasts, chese, jam, cream, fruit salad, fresh juice and coffee for 15 AR$.

If you prefer to sit down for some international cuisine, you can find Chinese, Thai, Middle Eastern or Japanese restaurants that will cost you just a little more.

Cumana Empanada is a great empanada/calzone/pizza place a half-block north of Av Santa Fe, on Rodriguez Pena St. Dinner or lunch for two will run you a total of around $ 10 (without beer or wine). Not romantic, it's a hole in the wall usually filled with young local college-age or young professionals. You can get a great calzone which won't fit on a large platter for around US$ 6 .